[gmx-users] energy minimisation of carbon nanotubes.

[Elie M]

Dear all,
I have a small question regarding energy minimization. I am minimizing a carbon naotube in vaccuo and my (mdp )input file is: ;

;     SWNT (10,0) on its own

;     Elie Moujaes, SWNT+polymers project

;     Energy minimization input file

;

cpp                 =/usr/bin/cpp

define              = 
-DFLEXIBLE

constraints         = 
none

integrator          = 
cg

nsteps              = 
2000

;

;     Energy minimizing stuff

;

emtol               =  0.00001

emstep              = 
0.03

 

nstcomm             = 
1

pbc                 =  xyz

periodic_molecules  =  yes

ns_type             = 
grid

rlist               =  1

rcoulomb            =  1.0

rvdw                =  1.0

Tcoupl              = 
no

Pcoupl              = 
no

gen_vel             = 
no
I have put emstep very loww toa chieve accuracy.In the output i got:
Reading file SWNT-EM.tpr, VERSION 4.0.5 (single precision)Polak-Ribiere Conjugate Gradients:   Tolerance (Fmax)   =  1.00000e-05   Number of steps    =         2000   F-max             =  3.48055e+03 on atom 799   F-Norm            =  9.67779e+02

Stepsize too small, or no change in energy.Converged to machine precision,but not to the requested precision Fmax < 1e-05
Double precision normally gives you higher accuracy.
writing lowest energy coordinates.
Polak-Ribiere Conjugate Gradients converged to machine precision in 359 steps,but did not reach the requested Fmax < 1e-05.Potential Energy  =  8.7646348e+03Maximum force     =  1.6186214e+01 on atom 701Norm of force     =  5.4860659e+00
It seems that I have to use the double precision version but as the single precision is concerned,Is the minimization fine (as the maximum force on atom 701 is 16...)?
Thanks 
Elie